1. Field of the Invention
The present invention relates to the art of electrochemical cells. More particularly, the present invention relates to a new cathode material for use in a high energy density electrochemical cell, preferably comprising silver and vanadium. The new cathode material has the general formula Ag.sub.x V.sub.2 O.sub.y and can comprise a .beta.-phase silver vanadium oxide having in the general formula x=0.35 and y=5.18 and a .gamma.-phase silver vanadium oxide having in the general formula x=0.74 and y=5.37 or a mixture of the phases. This new cathode material exhibits reduced voltage delay during high rate applications, such as when the cathode mixture is incorporated into a primary lithium electrochemical cell powering an implantable cardiac defibrillator.
2. Prior Art
Mixed metal oxides such as silver vanadium oxide are known for use as a cathode active material in an electrochemical cell. Keister et al. U.S. Pat. No. 4,830,940, which is assigned to the assignee of the present invention and is incorporated herein by reference, describes a nonaqueous lithium electrochemical cell comprising a silver vanadium oxide cathode material having the general formula Ag.sub.x V.sub.2 O.sub.y wherein "x" is in the range from about 0.5 to about 2.0, preferably from about 0.95 to about 1.1 and "y" is in the range from about 4.5 to about 6.0, preferably from about 5.0 to about 6.0. This mixed metal oxide material includes the .gamma.-phase silver vanadium oxide having the stoichiometric formula Ag.sub.0.74 V.sub.2 O.sub.5.37. However, Keister et al. does not disclose the .beta.-phase silver vanadium oxide. There is also no suggestion of a mixture of the .beta.- and .gamma.-phases of the cathode active material having x&lt;0.5. Finally, this patent does not address the problem of providing a cathode active material having a reduced voltage delay.
Weiss et al. U.S. Pat. No. 5,180,642 provides a cathode comprising manganese dioxide (MnO.sub.2), carbon monofluoride (CF.sub.x) or a mixture of the two and a cathode additive, such as silver vanadate having the formula .beta.-Ag.sub.x V.sub.2 O.sub.5, .delta.Ag.sub.x V.sub.2 O.sub.5 or Ag.sub.2 V.sub.4 O.sub.11 incorporated into the cathode mixture. The stoichiometric value of "x" is not further defined and there is no mention of the use of a mixture of the .beta.- and .gamma.-phases of the cathode active material. Also, Weiss et al. was not concerned with reduced voltage delay during high rate applications, but rather, the cathode additive serves as an end-of-service indicator.